Water in water supply and drainage pipe paths is pressurized to a desired pressure in a pressurization plant and then fed through water supply and drainage pipes to a destination where water pressure is inefficient. Pressure which is applied to the water in the pressurization plant is determined considering a distance between the pressurization plant and consumer, diameter or conditions of the water supply pipe etc. For example, in a case where a region to be supplied with the water is quite remote from the pressurization plant or includes hilly areas, the pressure applied to the water is relatively higher than a different case.
Meanwhile, when for example higher water pressure than necessary occurs in the water supply and drainage pipes, the water supply and drainage pipes and consumer-side water supply and drainage facilities may be damaged; therefore, a pressure relief valve for lowering the water pressure is almost essentially installed. In a case where the water pressure is higher than necessary, the pressure relief valve operates to allow the water pressure on a downstream side of the pressure relief valve to be kept almost constant, thereby preventing burst or water leak of the water supply pipe etc. for example.
The pressure relief valve has many types; for example, the pressure relief valve has been developed of a type which can generate electricity by using the pressure of water passing through the water supply and drainage pipes.
FIG. 1 is a view schematically showing a conventional pressure relief device to which a generation system is applied.
The pressure relief device illustrated in FIG. 1 includes a rotor (17) which is rotatable in a water supply pipe (11) with a shaft (21) as a rotational shaft; guide slopes (13, 15) fixed on the upstream and downstream sides of the rotor (17), respectively; and a generator(not illustrated) which generates electricity by receiving rotational torque from the rotor (17).
The rotor (17) has a plurality of blades (19) for rotating the shaft (21) by receiving the water pressure. The blades (19) serve to rotate the rotor (17) by receiving kinetic energy of the water passing through the water supply pipe (11).
The shaft (21) extends with its ends to the outside of the water supply pipe (11) to drive the generator installed outside the water supply pipe (11), thereby enabling the generator to generate the electricity.
Furthermore, the guide slopes (13, 15) are a protruding part fixed to an inner wall surface of the rotor (17) and guide streamlines of the water in a direction indicated by arrow “a” to allow the water to pass through a lower side of the rotor (17) as seen in the figure.
The reason why the pressure relief device having the above configuration reduces the water pressure is that the pressure relief device itself obstructs water flow by reducing sectional area of flow in the water pipe (11), thereby acting as an obstructor which lowers flow speed.
However, the conventional pressure relief device having the above configuration has a disadvantage that it cannot adjust the degree of pressure relief. That is, it cannot drop the water pressure downstream of the rotor (17) as desired. This is because the rotor (17) merely generates the electricity while rotating by receiving the kinetic energy of the water and, once starting to rotate, cannot act as a resistance body in a flow field any more.
Furthermore, the conventional pressure relief device has a structure where the water easily passes the rotor (17) while rotating the rotor due to actions of the guide slopes (13, 15); therefore, function of the rotor as the resistance body is further lowered.
In addition, the conventional pressure relief device described above does not have so good electricity generation efficiency. As illustrated, though the guide slopes (13, 15) guide the streamline of water in the direction indicated by arrow “a”, 100% of the water does not pass through the lower side of the rotor (17), and part of the water passes in a direction indicated by arrow “b” opposite to a rotational direction of the rotor (17), thereby impeding the rotation of the rotor.